The present invention relates to an energy filter for selectively passing only charged particles having a given energy and to an electron microscope using such an energy filter.
In a transmission electron microscope (TEM), an electron beam is directed to a specimen. The image created by the electron beam transmitted through the specimen is enlarged and projected onto a fluorescent screen. Thus, a magnified image of the specimen is obtained on the fluorescent screen. In recent years, an electron microscope has been developed which has an energy filter such as an xcexa9-filter mounted within the electron optical system for projecting the electron beam transmitted through a specimen onto the fluorescent screen. In this instrument, a TEM image is created from only those of transmitted electrons affected by the specimen which have passed through the xcexa9-filter.
In this electron microscope equipped with this xcexa9-filter, the energy spread of the electron beam directed to the specimen must be narrowed in order to enhance the energy selectivity (energy resolution) of the obtained TEM image. Therefore, in this kind of electron microscope, a cold-type field emission gun (FEG) that provides the narrowest energy spread today is adopted.
However, with this type of field emission gun, the obtained energy spread is about 0.4 eV, for example, at an accelerating voltage of 200 kV. Currently, it is difficult to narrow the energy spread further. On the other hand, in order to obtain a higher energy resolution, it is desired to suppress the energy spread of the electron beam impinging on the specimen to approximately 0.1 to 0.2 eV or less. Accordingly, a new attempt has been recently made to narrow the energy spread. In particular, an energy filter is mounted between an electron gun and a specimen. A slit is used to extract a narrow energy range from an energy spread created by an electron source. This electron beam having a narrow energy spread is directed to the specimen.
However, a practical energy filter having a resolution sufficient to extract electron beams with energy spreads of approximately 0.1 to 0.2 eV from electron beams with an accelerating energy greater than 200 kV (at which electrons have an energy of 200 keV) required by a high-resolution electron microscope is not available today. Accordingly, a method of mounting an energy filter at a position where the electron beam is in a low energy state to perform an energy selection and then accelerating the electrons to impart high energy has been discussed.
Where a low-energy electron beam passes through an energy filter in this way, the Boersch effect must be taken into consideration. That is, the velocities (or energies) of electrons (generally, charged particles) traveling close to each other affect each other due to Coulomb interactions. The Boersch effect becomes stronger with increasing either the current density or the electron velocity. Therefore, where a low-energy electron beam passes through an energy filter, especially if electrons travel close to each other in passing across the focal point in the filter, the electrons affect each other, varying the energies. In this way, energy broadening occurs. This makes it difficult to obtain an electron beam having a narrow energy spread.
It is an object of the present invention to provide an energy filter capable of suppressing the Boersch effect.
It is another object of the present invention to provide an electron microscope using this energy filter.
An energy filter that achieves the first-mentioned object in accordance with the teachings of the present invention has electric and magnetic fields that are combined to pass only charged particles having a given narrow energy range. Charged particles passed through this energy filter are focused plural times (i.e., at plural focal points) in the X direction perpendicular to the direction of travel of the charged particles. The charged particles are focused in the Y direction perpendicular to the X direction at a position different from the focal points in the X direction. An energy-selecting slit is mounted at the second or subsequent focal point in the X direction.
An electron microscope, in accordance with the present invention, has an electron gun for producing an electron beam and an energy filter for passing only electrons having a given narrow energy range to a specimen. The energy filter has electric and magnetic fields that are combined to pass only electrons having the given energies. Charged particles passed through this energy filter are focused plural times (i.e., at plural focal points) in the X direction perpendicular to the direction of travel of the charged particles. The charged particles are focused in the Y direction perpendicular to the X direction at a position different from the focal points in the X direction. An energy-selecting slit is mounted at the second or subsequent focal point in the X direction.
Other objects and features of the invention will appear in the course of the description thereof, which follows.